EP0307262B1 - Process for stirring steel in the ladle with carbon dioxide - Google Patents

Abstract

Process for stirring a stilled steel in a ladle, with the aid of carbon dioxide gas. <??>Before the stirring begins in the ladle, there is added to the usually employed quantity of deoxidiser an additional quantity of deoxidiser in the molten metal, the flow rate of carbon dioxide, allowing for the ladle capacity and the stirring time, remaining lower than or equal to the maximum flow rate corresponding to the oxidation of the additional quantity of deoxidiser at the end of stirring.

Description

The present invention relates to a method of stirring in a ladle ladle, in which an inert gas is injected into a bath of molten steel, this injection being carried out so that bubbles of gas rise through at least part of the bath. of molten metal and burst on the surface thereof, so as to create a setting in motion of the molten metal during the rise of the gas, said steel bath having been previously calmed for incorporation of a deoxidizer in quantity sufficient for an excess of it to remain dissolved in the bath.

In order to improve productivity and quality, steelmakers have developed so-called secondary metallurgy or pocket metallurgy. The main goal of this metallurgy is thermal control and analytical control of the metal. In terms of thermal control, stirring allows cooling and homogenization. At the level of analytical control, stirring makes it possible to achieve homogenization, nuancing of steel, deoxidation, control of the cleanliness of the metal, control of inclusions, desulfurization, dephosphorization, etc. It has also been found that the use of electric arcs to carry out reheating in a pocket for example, or of a vacuum to carry out degassing in this same pocket was improved by stirring the metal. Among the different brewing methods used, brewing by gas injection is widely used because it requires little investment and is very easy to use.

Before stirring, the effervescent steel is calmed by incorporation of deoxidizer such as aluminum and / or silicon, makes it possible to eliminate or reduce the residual oxygen present in the steel bath. In order to maintain a dissolved oxygen content in the steel compatible with the casting conditions, an excess of deoxidizer is generally incorporated in the steel bath. This excess deoxidizer is generally less than 1500 ppm, and preferably between 100 and 500 ppm for aluminum and between 200 and 1000 ppm for silicon. Depending on the desired grade of the steel, the content of dissolved deoxidizer is fixed and controlled at approximately ± 20 ppm.

The mixing corresponds to a setting in motion by entrainment of the metal during the rise of the gas. The intensity of the brewing is characterized by a physical quantity corresponding to the power per tonne of metal.

It is known to use neutral gases such as argon, or nitrogen, to carry out stirring in a pocket. In a certain number of applications, nitrogen cannot be used because the production of steel having a low nitrogen content is sought. Until now, only argon could be used for the gas mixing of the pockets when it is particularly desired to obtain steels with low nitrogen content. However, the use of argon is sometimes limited by economic constraints given the high cost of this gas.

We therefore investigated whether it was possible to use another gas to carry out this mixing, which exhibits a substantially inert behavior with respect to the steel while being economical during its use.

A priori, the person skilled in the art tends to rule out the possibility of using carbon dioxide gas to carry out bag mixing because it is known from the article entitled "Emprego de CO₂ na Descarburaçao do Aco em Formo Elétrico - Renato Augusto Barbosa da Silva - Getulio Sergio da Silva - METALURGIA - vol. 28 - N ° 172 - MARCO, 1972 "that carbon dioxide at the temperature of a bath of molten steel, ie of the order of 1,600 ° C decomposes into oxygen and carbon monoxide which have an oxidizing behavior with respect to steel.

Unexpectedly, it was found that it was possible to use carbon dioxide to carry out the stirring in a ladle of calm steel, despite the oxidizing nature of carbon dioxide under the conditions of use, while brewing economically.

The method according to the invention is characterized in that, before brewing in the ladle begins, an additional amount of deoxidizer is added to the excess of deoxidizer in the molten metal bath and then carried out stirring of the molten metal by injection of carbon dioxide in gaseous form, the rate of carbon dioxide in gaseous form, taking into account the capacity of the ladle and the stirring time, remaining less than or equal to the maximum rate corresponding to l oxidation of the additional amount of deoxidizer. Preferably, the additional amount of deoxidizer will be less than or equal to 10% of the excess deoxidizer. It was found that this value of 10% was the maximum value making it possible to control the content of deoxidizer in the steel according to the predetermined grade. The flow carbon dioxide per tonne of steel stirred is generally less than or equal to 10 liters per minute.

In-depth studies have made it possible to highlight the factors which influence the loss of deoxidizer during mixing, this deoxidizer being generally very reactive with respect to the iron oxide which surrounds the gas bubbles, thus causing the formation of oxides. . However, deoxidizers have a very high cost and one of the aims of the invention is to inject carbon dioxide under certain conditions so as to stir the molten metal, causing a loss of deoxidizer, the cost of which remains less than the economy achieved by the use of carbon dioxide, lower cost than argon. In addition, it is found that, unexpectedly, although oxides are produced in the metal during mixing with carbon dioxide, these do not cause deterioration in the cleanliness of the finished product.

Thus, we were able to show the importance of the following parameters during the stirring of a steel with a gas: the nature of the steel stirred, i.e. the composition aimed at the end of stirring, the nature and the quantity of deoxidizer used, at the start of brewing as well as the quantity of deoxidant required for casting after treatment in the ladle, the dimensions of the ladle (height, diameter) and the amount of metal treated, the type of gas injector used and its hydraulic characteristics, the gas used, the flow injected and the duration of the treatment.

The additional quantity of deoxidizers to be added to the steel before stirring must be able to be determined as a function of the geometry of the ladle, the duration of stirring in this ladle and the flow rate of carbon dioxide used.

relation dans laquelle:
B est le rapport entre la longueur de lance immergée dans le bain et la hauteur de métal dans la poche,
Q est le débit d'anhydride carbonique en litre par minute,
W est la capacité de la poche en tonne,
t est le temps de brassage en minute.According to a first preferred variant embodiment of the process of the invention, in which a lance is used to inject carbon dioxide gas into the bath, the process is characterized in that the flow rate Q of carbon dioxide gas is such that the following relationship is checked:
$${\text{B}}^{\text{1.42}}\text{× (}\frac{\text{Q}}{\text{W}}{\text{)}}^{\text{0.66}}\text{× t ≦ 35}$$

relationship in which:
B is the ratio between the length of lance immersed in the bath and the height of metal in the pocket,
Q is the carbon dioxide flow rate in liters per minute,
W is the capacity of the bag in tonnes,
t is the brewing time in minutes.

Do étant la teneur visée en désoxydant en fin de brassage exprimée en %, R étant le rendement d'addition du désoxydant de calmage exprimé en %, B étant le rapport entre la profondeur immergée de la lance et la hauteur de métal,
Q étant le débit en d'anhydride carbonique en litre par minute,
W étant la capacité de la poche en tonne,
t étant le temps de brassage en minute.In this case, the additional quantity m _sup. (expressed in kg) of deoxidizer to be added to the bag before mixing is less than or equal to:
$${\text{m}}_{\text{sup}}\text{≦ 3 ×}\frac{\text{Do}}{\text{R}}{\text{× B¹}}^{\text{, 42}}{\text{× Q⁰}}^{\text{.66}}{\text{× W}}^{\text{0.36}}\text{× t}$$

Do being the target deoxidizer content at the end of stirring expressed in%, R being the addition yield of the calming deoxidizer expressed in%, B being the ratio between the immersed depth of the lance and the height of metal,
Q being the flow rate of carbon dioxide in liters per minute,
W being the capacity of the bag in tonnes,
t being the brewing time in minutes.

formule dans laquelle:

• ― Q est le débit de gaz injecté en l/mn
• ― W est la capacité de la poche en tonne
• ― S est la surface active du bouchon en contact avec l'acier en cm²
• ― t est le temps de brassage en minute.

According to a second preferred embodiment of the invention, in which a porous plug is used to inject gaseous carbon dioxide into the molten metal bath, the method is characterized in that the flow rate Q of carbon dioxide is such that the following relation is verified:
$${\text{Q⁰}}^{\text{, 25}}{\text{× W⁻⁰}}^{\text{.64}}{\text{× S⁰}}^{\text{, 33}}\text{× t ≦ 10}$$

formula in which:

• - Q is the gas flow injected in l / min
• - W is the capacity of the bag in tonnes
• - S is the active surface of the plug in contact with the steel in cm²
• - t is the brewing time in minutes.

formule dans laquelle:
Do est la teneur visée en désoxydant en fin de brassage exprimée en %,
R est le rendement d'addition du désoxydant de calmage exprimé en %,
Q est le débit d'anhydride carbonique exprimé en litre/minute,
W est la capacité de la poche en tonnes,
t est le temps de brassage en minute,
S est la surface active du bouchon poreux en contact avec l'acier exprimée en cm².In the case of a porous plug, the additional quantity m _sup of deoxidizer to be added to the molten metal bath is equal to:
$${\text{m}}_{\text{sup}}\text{≦ 4 ×}\frac{\text{Do}}{\text{R}}{\text{× Q⁰}}^{\text{, 25}}{\text{× W⁰}}^{\text{, 36}}{\text{× S⁰}}^{\text{, 33}}\text{× t,}$$

formula in which:
Do is the target deoxidizer content at the end of brewing expressed in%,
R is the addition yield of the calming deoxidizer expressed in%,
Q is the carbon dioxide flow rate expressed in liters / minute,
W is the bag capacity in tonnes,
t is the brewing time in minutes,
S is the active surface of the porous plug in contact with the steel expressed in cm².

formule dans laquelle,
Q est le débit d'anhydride carbonique exprimé en litre par minute,
W est la quantité de métal traité dans la poche, exprimée en tonne,
t est le temps de brassage en minute,
S est la section mouillée en cm² qui dans le cas de tubes circulaires est égale à:

tandis que dans le cas de rainures ou de tubes applatis:
$$\text{S = N × (L + 0,05) × (l + 0,05),}$$

N étant le nombre de passages élémentaires sur un injecteur,
d étant le diamètre intérieur du tube en cours,
L et l étant respectivement la plus grande longueur et la plus grande largeur de la rainure exprimées en cm.According to a third preferred embodiment of the invention, in which the gas is injected into the pocket using an injector in which the gas passes through a space formed between the non-porous refractory blocks, the passage section gas being controlled either by grooves in the refractory blocks or preferably by a series of metal tubes of small diameters and of circular or flattened section, the process is characterized in that the flow Q of carbon dioxide in the metal is such that the following relation is verified:
$${\text{Q⁰}}^{\text{, 25}}{\text{× W⁻⁰}}^{\text{.64}}{\text{× S⁰}}^{\text{, 33}}\text{× t ≦ 7}$$

formula in which,
Q is the carbon dioxide flow rate expressed in liters per minute,
W is the quantity of metal treated in the ladle, expressed in tonnes,
t is the brewing time in minutes,
S is the wetted section in cm² which in the case of circular tubes is equal to:

while in the case of grooves or flattened tubes:
$$\text{S = N × (L + 0.05) × (l + 0.05),}$$

N being the number of elementary passages on an injector,
d being the inside diameter of the current tube,
L and l being respectively the greatest length and the greatest width of the groove expressed in cm.

In the case of injection using injectors as defined above, the additional quantity m _sup of deoxidizer to be added to the molten metal is given by the same formula as in the case of porous plugs, the surface S then being calculated according to one or other of the formulas mentioned above.

The invention will be better understood with the aid of the following exemplary embodiments, given without implied limitation:

Example 1 .

Brewing is carried out in a 180-tonne pocket using a lance, immersed at three-quarters of the height of the bath of molten steel. This stirring is carried out using a carbon dioxide gas flow rate of 200 liters per minute for 8 minutes. The addition yield R of aluminum is 50%. The aluminum content targeted at the end of brewing is 0.02%.

The amount of additional aluminum calculated m _sup is equal to 1.37 kg.

By adding this additional quantity of aluminum before stirring carried out as indicated above, it is verified by carrying out an analysis of a sample taken at the end of stirring that the aluminum content of the steel is indeed 0.02% ( 200 ppm).

Example 2 .

The same experiment is carried out as in the case of Example 1, using a porous plug placed in the bottom of the bag, the active surface of which is 190 cm 2.

The additional quantity m _sup of aluminum to be added, calculated according to the formula mentioned above, is equal to 1.76 kg.

By carrying out the stirring according to the indications given above by adding before the start of stirring the amount of 1.76 kg of aluminum in the steel bath, it is found by analysis of a sample taken from the bath at the end of stirring that the aluminum content of the sample is indeed 0.02% (200 ppm).

Example 3 .

Is carried out under the same conditions as above with stirring using an injector consisting of tubes of small diameters whose equivalent diameter does not exceed 3 mm. We use a section equal to 0.7 cm².

The quantity of aluminum m _sup to be added, calculated according to the formula mentioned above is 1.27 kg. By carrying out the stirring according to the indications given above, it is verified that a sample taken at the end of stirring does indeed contain an aluminum content equal to 0.02% (200 ppm).

In general, it will be noted that during the processing of the metal in the ladle according to the method described above, it may prove to be preferable or necessary to carry out inerting of the surface of the steel bath during the entire brewing time. In particular, this may prove necessary if one wishes to keep the nitrogen content of the treated steel low. This inerting may be carried out by injection of argon, nitrogen (when this is not to be excluded) or carbon dioxide above or on the surface of the bath. For the first two gases mentioned, this inerting can be carried out using gas or liquid. For carbon dioxide, this inerting can be carried out using gas or carbon dioxide snow.

Claims (7)

1. Process for stirring killed steel in the ladle, in which process an inert gas is injected into a bath of molten steel, this injection being carried out so that the bubbles of gas rise through at least a part of the bath of molten metal and burst at the surface of this bath, so as to create a setting in motion of the molten metal when the gas rises, the said bath of steel having been killed beforehand by the incorporation of a deoxidiser in a sufficient quantity so that an excess thereof remains in the dissolved state in the bath, characterised in that, before stirring in the ladle begins, there is added to the excess of deoxidiser in the bath of molten metal an additional quantity of deoxidiser, and in that stirring of the molten metal is subsequently carried out by the injection of carbon dioxide in gaseous form, with the delivered quantity of carbon dioxide in gaseous form, taking into account the capacity of the ladle, remaining less than or equal to the maximum quantity delivered corresponding to the oxidation of the additional quantity of deoxidiser.

2. Process according to claim 1, characterised in that the additional quantity of deoxidiser is less than or equal to 10% of the dissolved excess of deoxidiser.

3. Process according to claim 1 or 2, in which a lance is used for injecting the gaseous carbon dioxide into the bath of molten metal, characterised in that the quantity Q of gaseous carbon dioxide delivered is such that the following equation is valid:
$${\text{B}}^{\text{1.42}}\text{ × (}\frac{\text{Q}}{\text{W}}{\text{)}}^{\text{0.66}}\text{ × t ≦ 35}$$

in which:
B is the relationship between the length of lance immersed in the bath and the level of the metal in the ladle,
Q is the quantity of carbon dioxide delivered, in litres per minute,
W is the capacity of the ladle in tonnes;
t is the time of stirring in minutes.

4. Process for stirring in the ladle according to claim 1 or 2, in which a porous plug is used which is placed in the lower wall of the ladle for injecting the gaseous carbon dioxide into the bath, characterised in that the quantity Q of gaseous carbon dioxide delivered is such that the following equation is valid:
$${\text{Q}}^{\text{0.25}}{\text{ × W}}^{\text{-0.64}}{\text{ × S}}^{\text{0.33}}\text{ × t ≦ 10}$$

in which:
Q is the quantity of injected gas delivered, in l/mn,
W is the capacity of the ladle in tonnes,
S is the active surface area of the plug in contact with the steel in cm²,
t is the time of stirring in mn.

5. Process for stirring in the ladle according to claim 1 or 2, in which injectors are used for injecting gaseous carbon dioxide into the bath of molten metal, characterised in that the quantity Q of gaseous carbon dioxide delivered is such that the following equation is valid:
$${\text{Q}}^{\text{0.25}}{\text{ × W}}^{\text{-0.64}}{\text{ × S}}^{\text{0.33}}\text{ × t ≦ 7}$$

in which:
Q is the quantity of carbon dioxide delivered, expressed in litres per minute,
W is the quantity of metal treated in the ladle, expressed in tonnes,
t is the time of stirring in mn,
S is the wetted section in cm² which, in the case of circular pipes, is equal to:

whereas, in the case of grooves or flat pipes:
$$\text{S = N × (L + 0.05) × (l + 0.05),}$$

N being the number of elementary passages on an injector,
d being the internal diameter of the pipe in cm,
L and l being, respectively, the greatest length and the greatest width of the groove, expressed in cm.

6. Process for stirring in the ladle according to claim 3, characterised in that the additional quantity of deoxidiser m_sup to be added is less than or equal to:
$${\text{m}}_{\text{sup}}\text{≦ 3 × }\frac{\text{Do}}{\text{R}}{\text{ × B}}^{\text{1.42}}{\text{ × Q}}^{\text{0.66}}{\text{ × W}}^{\text{0.36}}\text{ × t}$$

Do being the desired content of deoxidiser at the end of stirring, expressed as a percentage,
R being the addition yield of aluminium for killing, expressed as a percentage,
B being the relationship between the depth of immersion of the lance and the level of metal,
Q being the quantity of carbon dioxide delivered, in litres per minute,
W being the capacity of the ladle in tonnes,
t being the time of stirring in minutes.

7. Process for stirring according to claim 4 or 5, characterised in that the additional quantity of deoxidiser to be added is less than or equal to:
$${\text{m}}_{\text{sup}}\text{≦ 4 × }\frac{\text{Do}}{\text{R}}{\text{ × Q}}^{\text{0.25}}{\text{ × W}}^{\text{0.36}}{\text{ × S}}^{\text{0.33}}\text{ × t,}$$

in which:
Do is the desired content of deoxidiser at the end of-stirring, expressed as a percentage,
R is the addition yield of killing deoxidiser, expressed as a percentage,
Q is the quantity of carbon dioxide delivered, expressed in litres/minute,
W is the capacity of the ladle in tonnes,
t is the time of stirring in minutes,
S is the active surface area of the porous plug in contact with the steel, expressed in cm².